[Asterisk-Users] yet another question on DID trunks

[Steven Critchfield]

On Wed, 2004-01-07 at 13:38, john lawler wrote:
> Hey Steven,
> 
> Sorry to bother you yet again w/ a question on my seemingly endless 
> quest to get DID trunks setup for a customer.
> 
> If you don't know anything about this issue or would rather I looked 
> elsewhere (including the Asterisk list, I suppose), please just let me 
> know right off the bat.  I'm having great difficulty finding good 
> resources on the web that explain this technology.

So either my demeanor has changed a bit post holidays, or my reputation
is harsher than it should be. Either way, here is my response. 

> I'm still trying to examine a DID solution for this customer but don't 
> understand how a single trunk (whatever that is--I assumed just a single 
> pair of wires like a POTS line, but I'm thinking now it must not be) can 
> support multiple (incoming) phones calls.  I've also been told how many 
> phone calls per trunk depends on your equipment.  Do you know anything 
> about equipment that interfaces with DID trunks?  Are there special 
> devices that do this?  I think I've heard about FXS port devices 
> supporting DID.  But how could a single FXS port (say on a channel bank, 
> or even one of the Digium PCI FXS cards) support multiple 
> conversations?  Asterisk would certainly just see it as a single channel.

Unfortunately there seems to be confusion even among some telecom sales
people or installers on some technology. In the phone lines, you have a
circuit, otherwise known as a phone line. This circuit may or may not
have a phone number assigned to it. A while back when we had a T1 from
XO(I think) they wouldn't assign numbers to specific lines if we had
DIDs because it meant we would use too many phone numbers. Before that
our T1 had a number assigned to each line, but had it broken down to two
hunt groups. 

Anyways, moving forward with your question. It is unlikely that a single
POTS line would have DID on it. In this case a trunk may mean a T1. This
is especially likely if you are being told that the trunk supports
multiple calls at one time. 

It is also possible that you are referring to a Centrex line. A Centrex
line allows the telco switch become your PBX for you. It does a lot of
the normal PBX functionality and even allows you to do transfers even
though that then means you are switching the circuit the call is
completed on.

DID can be accomplished using E&M wink. E&M helps eliminate potential
collisions in calls. (Ever pick up the phone to dial out and just
happened to answer an incoming call before the first ring?) In this
method, the telco signals a incoming call, the PBX answers the line and
listens to the pulse, DTMF, or MF tones played, and when they match an
incoming extension it will wink(quick hook cycle) the line as an
acknowledgment . At this point the PBX will answer the line for real,
and proceed to process the call as normal. The incoming digits are
usually the last 3 or 4 digits of the called number to keep that part of
call processing short. AFAIK, you don't find DID on analog lines much if
at all. 

ISDN is also capable of acting like a DID line in that it can have
multiple numbers assigned to any of the B channels and it can signal
what number was called as it completes the call. This is for BRI and
PRI. 

So if you are speaking of a T1 interface, you can use a standard T100P
or E100P or any of the quad cards to make the connection. Asterisk
supports DIDs by just placing an extension entry with the exact match
for what the telco sends you in whatever context you define as receiving
calls from that interface. 

> Anyway, those are the questions I've got at the moment based on my 
> (lacking) understanding of how DID (and other) trunks operate.  I've got 
> two Digium T1 cards, a Rhino Equipment FXS channel bank and a Carrier 
> Access channel bank w/ 2 - 12 port FXO cards in it.  And I'm just about 
> ready to start testing that system, but I want to determine the 
> additional difficulty + cost of using DID trunks for inbound calls 
> instead of POTS lines.  Oh, and the other disclaimer is, fractional T1 
> is not an option for me, so these'd have to be "analog" DID trunks, or 
> whatever the default way of receiving them is.

I don't think analog DID is going to be an option for you either. You
need to either do a good survey of the currently deployed interfaces or
have a long talk with the telco sales person about what they offer. If
you can get it on a T1 then you remove the AB1 with the FXO cards in it.
Also you will be less likely to have echo and any modem connections
behind asterisk are more likely to work. 
-- 
Steven Critchfield  <critch at basesys.com>